Trained Eye


Mould in Basement



I have a question about mould in our basement.  We live in a 25 year old home with a partially finished and insulated basement.  I have noticed that the person who did the work put the insulation bare against the foundation.  Right now there is frost forming on the foundation walls on the inside of the basement.  I noticed quite a bit of dark coloured build up on the wall particularly in one corner of the basement.  My question is how do I know that this is mould, can I destroy it or should it be done professionally? Finally, should the insulation be taken down and a vapour barrier placed against the foundation wall in the basement?  I have two young children who regularly play in the basement, so I am concerned.

Thanks for any advice.


The issue of mould in homes is a very timely one and we may all be inundated with press coverage in the coming few years on this topic. The reason that mould is in the forefront is due to the tightening of our homes and building envelopes for energy efficiency.

In striving to make homes more energy efficient over the last 20 to 25 years, we have significantly reduced the amount of heat loss from our homes. This has been accomplished by highly insulating our walls and attics, upgrading windows and doors, and sealing gaps in all these areas. All these improvements have dramatically reduced the amount of air movement out of the home. Less air movement out of the home, less heat loss. The problem with this scenario is: less air movement out also means less fresh air movement into the home.

In many cases, this situation has created another set of completely different problems. Preventing stale house air from escaping the home and fresh outside air from entering traps many pollutants and other airborne substances in the home. Many of these substances are created from everyday living, washing, cooking, and cleaning. Home occupants with sensitivities to chemicals or allergies have seen an increase in reactions in the tighter new homes.

One airborne substance that has been one of the most difficult to expel from the home is moisture. Water vapour is one of the main components of air and is easily trapped in a well insulated home with modern air-vapour barriers. Moisture levels are increased in a home through bathing, cooking, laundry, and human respiration. If too much moisture is present, it will condense on cooler surfaces such as windows and foundation walls. This moisture may dampen cellulose-based building materials and allow mould growth. If the temperature is low enough, frost, such as that seen in your home, will form. When the frost melts it will soak the wall and insulation, trapping the moisture in an area with little air movement. If this moisture is not allowed to quickly dry, with adequate air movement, mould may grow.

Determining if the dark coloration in the basement is mould may not be necessary to remove it, but the source of the moisture must be dealt with first. Installing and operating bathroom and kitchen exhaust fans, running furnace fans continually, shutting off furnace humidifiers and installation of a fresh air intake near the furnace may reduce the humidity in the home. Not allowing excess storage in the basement will also increase air movement in the corners, preventing condensation and mould growth. Opening basement windows in the warmer months and operating central air conditioners will also help dry out the damp basement. If none of these measures make a significant difference, then a mechanical ventilation system may be the ultimate solution.

Once the moisture level is reduced, and air movement increased at the basement walls, there should not be enough condensation on the cool foundation walls to form frost. It appears from the question above, that the insulation is not complete on the walls and a poor air-vapour barrier installation is likely. After the moisture level in the home is reduced, the insulation should be removed and dried and any mould washed from the foundation walls with a 10% bleach solution. If the mould is embedded in the insulation, wood framing or wall covering, they must be discarded.

Once everything is clean and dry, the insulation should be replaced and a proper polyethylene air-vapour installed on the warm side and sealed well with acoustical sealant. A second layer of poly should never be placed along the foundation wall. Installing the plastic sheathing on the cool side of the insulation will only trap moisture within the wall cavity, making the moisture problem and likelihood of mould growth worse. The source of the moisture and frost is not from the foundation wall, but from the home itself. The air-vapour barrier is installed on the warm side to prevent moist house air from entering the wall from the basement.

Testing of the “dark coloured build-up” in the corner of the foundation walls may be done, if desired, by a local laboratory. There are several in the local area that will instruct you in how to collect as sample and provide the swabs and information for collection. This should only be done to confirm what is almost certainly mould growth and may not be required if cleaning with a bleach solution removes the build-up. Certain individuals with allergies, low immune systems, and small children may be more sensitive to certain toxic moulds and quick action to reduce humidity and remove the mould is warranted.




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#110-2025 Corydon Ave
Winnipeg, MB
R3P 2G9